Alternating current generator field regulation control

ABSTRACT

A circuit configuration for controlling the field current of an alternating current generator in a vehicle electrical generator system includes a digital to analog converter for converting a digital command signal to an analog voltage. A first comparator compares the system field current with the converted analog voltage. A switched component, operatively connected to a power switch, is switchable in response to the comparator output. A frequency generator, operatively connected to the switched component, provides timed pulses to periodically tell the switched component to turn on the power switch.

TECHNICAL FIELD

This invention relates to alternating current (AC) generator regulatorsand more particularly to a field regulation control and method utilizinga fixed frequency “turn on” and a set point “turn off”.

BACKGROUND OF THE INVENTION

It is known in the art relating to alternating current (AC) generatorregulators to use a pulse width modulation (PWM) scheme where the PWM iscommanded by a non-linear algorithm to regulate the generator field. Thealgorithm is non-linear for load response reasons and to compensate forthe non-linear response of the field and hence the generator. Non-linearhere references the fact that a fixed current change (example 10 amps)will create a different transient depending on whether the generator isnear full load or is near no load. A free wheeling diode is typicallyused to clamp the inductive energy of the field. This results in afaster or slower decay in the freewheel mode than the field charge modedepending on whether the field is at high or low current respectively.The average voltage applied to the field is proportional to B+, whichcan cause some instabilities because increasing B+ causes field voltageto increase which in turn increases B+.

Conventional regulators also have an overcurrent circuit which shutsdown the output device for a finite period of time. This overcurrentcircuit has caused some problems when the field resistance gets lowwhich occurs when the generator is cold resulting in reduced generatoroutput until the overcurrent condition goes away.

SUMMARY OF THE INVENTION

An object of the present invention is to reduce alternating current (AC)generator field regulation circuit complexity and improve generatorregulator performance.

Another object of the invention is to eliminate the need for anon-linear control to compensate for the varying response at differentloads to a fixed load step. Still another object is to eliminate theover current problem in conventional designs.

Yet another object of the invention is to eliminate the need forovervoltage protection.

Accordingly, a field regulation control and method are provided whichuses current through the switch. A circuit configuration for controllingthe field current of an alternating current generator in a vehicleelectrical generator system includes a digital to analog converter forconverting a digital command signal to an analog voltage. A firstcomparator compares the system field current with the converted analogvoltage. A switched component, operatively connected to a power switch,is switchable in response to the comparator output. A frequencygenerator, operatively connected to the switched component, providestimed pulses to periodically tell the switched component to turn on thepower switch.

The circuit configuration further includes a second comparator forcomparing a sensed voltage, where voltage regulation is required, with areference voltage. An up/down counter converts the over/under signalfrom the comparator to the digital command signal. A frequency generatorincrements the up/down counter. A supervisory control circuit isoperatively connected to the up/down counter. Generator field and phasesignals, and an ignition input signal are used to power up and powerdown the generator.

A method for controlling the field current of an alternating currentgenerator in a vehicle electrical generator system includes:

converting a digital command signal to an analog voltage;

comparing the system field current with the converted analog voltage;

switching off a power switch in response to the comparator output; and

generating timed pulses to periodically tell the switched component toturn on the power switch.

The method further includes the steps of:

comparing a sensed voltage, where voltage regulation is required, with areference voltage;

converting the over/under signal from the comparator to the digitalcommand signal; and

incrementing the digital command signal.

These and other features and advantages of the invention will be morefully understood from the following description of certain specificembodiments of the invention taken together with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram schematically illustrating a vehicleelectrical generator system including a circuit configuration forgenerator field regulation control constructed in accordance with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 of the drawings, a circuit configuration for analternating current generator field regulation control is indicated byreference numeral 10 and is used to control the voltage in a vehicleelectrical generator system that uses a 3 phase alternator 12 with itsassociated field coil 14, stator coils 16 and diode bridge or rectifiers18. As is hereinafter more fully described the electrical generatorsystem includes a number of sub systems generally referred to as analternator and diode bridge subsystem, a field control power stagesubsystem, a field current controller subsystem, a voltage controllersubsystem, and a supervisory control subsystem.

With continuing reference to FIG. 1, the alternator 12 and diode bridge18 subsystem AD convert rotational energy into DC electrical energy. Avarying magnetic field is produced by the rotating electromagnetreferred to as field coil 14. Coil 14 includes wire wound around amagnetic material (usually iron) and the magnetic flux is directed tothe stator 16 through an air gap. Current is delivered to the field coil14 through a pair of slip rings and brushes 20, 22. The brushes 22 arestationary spring loaded contacts that make connection with the rotatingslip rings 20. The slip rings 20 are connected to the field windings andcurrent passes through the brushes 20 into the field. The rotatingmagnetic field is coupled to the stator 16 which includes multiplewindings (3 typically) that are evenly spaced in electricaldisplacement. (120 electrical degrees for a 3 phase winding). Thisconstruction produces alternating voltage and current waveforms that areevenly spaced electrically and provide relatively smooth power output.The alternating current (AC) is converted to direct current (DC) by thediode bridge 18. Current coming out of stator 16 is directed into thepositive leg of the bridge 18, and current going into the stator isdirected to take it out of the negative leg. Output capacitor 24 is usedto smooth out the transients caused by diode switching.

The field control power stage subsystem FCPS includes 3 basiccomponents. A power switch 26, such as a power MosFet, a switch driver28, a free wheeling or catch diode 30, and a shunt 32. The power switch26 is used to apply B+ voltage to the field at the command of thecontroller. B+ is supplied through the shunt 32 in order to give thecontroller information about field current. The shunt 32 can be externalor internal to the switch 26 and the shunt can be a magnetic pickupinstead of a resistive element. The switch drive 28 is used to convertlogic level signals to signals that the power switch 26 can use to turnon and off. The free wheeling diode 30 allows the current to circulatewhen the switch 26 turns off. When the switch 26 is turned on, the fieldcurrent will increase with the traditional single pole R/L response. (Atypical time constant is about 100 mS.) When the switch 26 is turnedoff, the field inductance does not want to change current and willproduce a negative voltage which is clamped by the free wheeling diode.When the switch 26 is operated at a fixed on to off ratio or duty cycle,this imposes an average voltage on the field proportional to B+ and theduty cycle. The single pole response of the field will give an averagecurrent through the field with a ripple content determined primarily bythe frequency of switching and the time constant of the field.

The field current controller subsystem FCC includes the circuitconfiguration for the alternating current generator field regulationcontrol, takes either a digital or analog current command and gives thepower stage the signals on when to turn on and off to regulate the fieldcurrent. A digital to analog converter (D/A) 38 converts the digitalcommand to an analog voltage that comparator 40 can use. The comparator40 determines if the current in the power switch 26 feeding the field isgreater than a command point and is used to tell flip flop 42 when toturn switch 26 off. A frequency generator 44 is used to periodicallytell the flip flop 42 to turn switch 26 on. This results in the turn onevent being driven by a fixed frequency, and the turn off event beingdetermined if the actual current exceeds the command set point. The flipflop 42 is edge triggered so that in the event of a fault (short) in thefield, high frequency oscillations are not set up as the set and resetsignals fight each other. The end result is a fixed frequency switchingsignal where the max current is the current command.

The voltage controller subsystem VC comprises four basic elements. Thepurpose of this subsystem is to compare a system voltage (sense voltage)to a reference voltage V_(Ref). The sense voltage is sensed at a pointwhere voltage regulation is desired, usually at the engine crankingmotor or at the vehicle battery. A voltage comparator 48 determines ifthe sensed voltage is higher or lower than the reference voltageV_(Ref). An up/down counter 50 converts the over/under signal from thecomparator 48 to a digital current command, and a frequency generator 52increments the up/down counter. The basic function is to increase thecurrent command by a preset rate when the voltage is under the setpoint, and to decrease the current when it is over the set point. Therate at which it increases or decreases is determined by the rampfrequency. The ramp frequency is set to give the desired systemdynamics, if it is too fast the overall system will oscillate, if it istoo slow, excessive over and undershoot will result when a load changeoccurs.

The supervisory control subsystem SC supervises the overall control ofthe circuit 10 and provides communication with the system (vehicle). Itprovides functions of power up and power down, generator disable, faultindication, and some performance indicators. Power up and down arecommanded by the system through either the ignition input 56 or thelight terminal 58. The ignition terminal is an input only and will powerup the generator as soon as it sees voltage. The light terminal 58 is abidirectional open collector type (or open drain) that will pull low tolight the indicator and also senses when voltage is applied to power upthe generator. The supervisory control SC will power down the generatorwhen the light and ignition signals are removed and the generator stopsrotating. The supervisory control SC senses generator speed through thephase signal. Some supervisors will respond to a set of input conditions(combinations of ignition, light, and other parameters) to disablegenerator output without turning the circuits off. This is typicallydone to temporarily remove the generator load from the engine. Settingthe up/down counter to 0 does this here. Field and phase signals areisolated with some resistance and presented to the system (vehicle) foruse in determining generator speed and load.

Changes in current command are executed as fast as the system willallow. The switch 26 will go full on until the current is met or it willgo off until the current decays to the desired value. In addition, itreduces the B+ sensitivity. When the field is current controlled, thecurrent never gets higher than the maximum. Since the turns in the fielddon't change, it not only makes it more robust to changes in fieldresistance, but full generator output can be obtained when it wouldnormally shut the generator down due to overcurrent. If the voltage rampfrequency is correctly set, voltage stability is very good with noovershoot. If it is ramped so that the current command is just ahead ofthe field current response (typically 100 ms time constant), then thefield current will go as fast as it can to the level that creates thedesired output voltage. Thus the need for overvoltage protection may beeliminated since the field switch is essentially shut off until thevoltage gets below the set point.

While the invention has been described by reference to certain preferredembodiments, it should be understood that numerous changes could be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedisclosed embodiments, but that it have the full scope permitted by thelanguage of the following claims.

What is claimed is:
 1. A circuit configuration for controlling the fieldcurrent of an alternating current generator adapted for use in a vehicleelectrical generator system, the circuit configuration comprising: adigital to analog converter to convert a digital command signal to ananalog voltage; a first comparator for comparing the system fieldcurrent with the converted analog voltage; a switched componentoperatively connected to a power switch and switchable in response tothe comparator output; a frequency generator operatively connected tothe switched component and providing timed pulses to periodically tellthe switched component to turn on the power switch; a second comparatorfor comparing a sensed voltage, where voltage regulation is required,with a reference voltage; an up/down counter for converting theover/under signal from the comparator to said digital command signal; afrequency generator to increment the up/down counter; and a supervisorycontrol circuit operatively connecting said up/down counter, saidgenerator field and phase signals, and an ignition input to power up andpower down said generator.
 2. The circuit configuration of claim 1wherein said switched component is a RS flip flop.
 3. A method forcontrolling the field current of an alternating current generatoradapted for use in a vehicle electrical generator system, the methodcomprising: converting a digital command signal to an analog voltage;comparing the system field current with the converted analog voltage;switching off a power switch in response to the comparator output;generating timed pulses to periodically tell the switched component toturn on the power switch; comparing a sensed voltage, where voltageregulation is required, with a reference voltage; converting anover/under signal from the comparator to the digital command signal;incrementing the digital command signal; and operatively connecting anup/down counter, a generator field and phase signals, and an ignitioninput to power up and power down said generator.